The present invention relates to an improved, energy efficient spray dryer utilized to transform liquid feed solutions, through the almost instantaneous evaporation of the liquid in the feed solutions, to dry, finely divided powders, and more particularly, to such a spray dryer having a novel and unique air distribution system.
Spray dryers have been used for a number of years to produce powdered milk, powdered chalk, powder for cosmetics, and other similar powdered products from feed solutions consisting of solids dissolved, dispersed or suspended in water. Essentially, all spray dryers are comprised of: a source of a continuous flow of hot air; a drying chamber; an atomizing system; and a powder recovery system, with the air source typically including an air supply fan, an air heater and an air distribution system, and with the powder recovery system typically including a powder separation system and an exhaust air flow system. The function of the air source is to cause heated air, flowing continuously at a relatively high velocity, to be introduced into the drying chamber while the feed solution is being dispersed as small droplets into the air flow by the atomizing system.
Over the years, those working in the art have recognized that the structure and arrangement of the air source, and particularly that of the air distribution system, are of prime importance with respect to obtaining the goals of the efficient evaporation of feed solution in the drying chamber and of the production of a high quality powdered product. A number of different air distribution systems have been utilized to attempt to achieve these goals. In one prior known air distribution system, the drying air flows, along a generally horizontal path, from the air supply fan into a larger distribution chamber, and past an air heater. The heated drying air then continues to flow along the horizontal path until it is introduced into the drying chamber through a vertically disposed, open ended cylindrical distributor tube. A plurality of horizontally disposed woven wire screens are mounted in the distributor tube, between its ends, to attempt to make the air velocity more uniform. A cooling ring is mounted about the lower end of the distributor tube, i.e. the end of the tube adjacent to the upper end or roof of the drying chamber. A small portion of the relatively cool air, flowing from the air supply fan, is directed to flow through the cooling ring and to exit from the ring, through an annular air gap, to the exterior of the spray dryer. Another known prior art distribution system is similar to the foregoing system except that the upper, inlet end of the vertically disposed distributor tube is cut off at an angle, normally 45.degree., with respect to the longitudinal vertical axis of the tube and that the horizontally disposed woven sire screens are positioned adjacent to the lower end of the tube. These two prior systems are described more specifically hereafter. Still another known prior art distribution system is similar to the first, above described systems except that the upper, inlet end of the vertically disposed distributor tube is flared outwardly and that the horizontal disposed woven wire screens are positioned adjacent to the upper inlet end of the distributor tube. In yet another known prior art distributor system, heated air is introduced into the drying chamber through a generally uniform diameter conduit. A bank of relatively small diameter tubes are disposed in and across the conduit downstream from the air heater. A venturi is positioned between this bank of small tubes and the drying chamber so that the air flowing from the venturi is introduced directly into the drying chamber. The end of the conduit adjacent the upper end of the drying chamber is cooled by the circulation of water through a ring disposed about the lower end.
While the foregoing prior art distribution systems perform adequately, by contemporary standards, it has been found that practical limits exist as to the air velocity distribution and the temperature of the air that may be introduced into the drying chambers of the systems and that these limits reduce the efficiency of the prior spray dryers. In other words, while ideally the temperatures of air should be as high as the particular product can tolerate and the velocity of the air should be as even in distribution as the system will produce, the structure and arrangement of the air distribution systems impose limits on the maximum temperature and velocity distribution of the air flow that can be used during the actual operating of the prior spray dryers. We have analyzed these prior art distribution systems and in our opinion, the practical limitation of these prior systems is caused by the fact that the air flowing into the drying chamber does not have sufficiently uniform velocity and temperature profiles. More specifically, each droplet emerging from the atomizer system should be dried with air at a predetermined desired temperature. In these prior air distribution systems, it has been found that while the average temperature of the air may be at or near this desired temperature, there will be significant variations in the temperature of air, as measured across a plane transverse to the air flow. The droplets subjected to the lower temperature air will be too "wet" and will deposit on the lower parts of the drying chamber. The droplets subjected to the higher temperature air will be too "dry" and will give rise to an inferior product quality.
The effect of variations in the velocity profile is similar to that of the temperature profile since the rate of drying of the droplet particles is proportional to the amount or volume of air introduced onto each droplet particle. To avoid the problems caused by the nonuniform velocity and temperature profiles, it has been necessary to operate the prior spray dryers at lower, less efficient temperatures and velocities.
Another problem present in the prior air distribution systems is the formation of a fringe of burned powder at the drying chamber roof--distribution system interface. The introduction of air into the drying chamber causes eddy currents in the chamber, and these currents in turn cause some partially dried droplet-particles to migrate back up onto the roof of the chamber where they are "burned on" and form the fringe of burned powder. This fringe is detrimental to the operation of the dryer in that parts of the fringe continually fall off and contaminate the powdered product and in some cases, the fringe will ignite and cause extensive burning in the drying chamber.
It is a primary object of our present invention to provide an improved spray dryer which has a novel air distribution system capable of producing relatively even or uniform air velocity and temperature profiles and which thus may be operated satisfactorily at air velocities at or relatively near the maximum velocities obtainable in the spray dryer and at drying air temperatures at or relatively near the maximum optional drying temperature for the product being processed. It is a related object of our present invention to provide an improved spray dryer of the type described wherein the longstanding above described problem of fringe formation at the drying chamber roof-distribution system interface is significantly reduced if not completely prevented.
More specifically, the improved spray dryer of our present invention includes a novel air distribution system wherein tapered or sloped transition ducting is used to direct the air from the air supply fan to the air heater. This transition ducting allows the air to expand evenly over the whole area of the heater. A pre-profile plate is mounted immediately upstream of the heater and functions to level out differences in the velocity of the air coming from the air supply fan. In other words, the pre-profile plate helps to even out air velocity differences in the air entering the heater, resulting in a more even temperature distribution from the heater. A profile plate is used with the heater to prevent large quantities of unheated air from bypassing the heater. Despite the use of the pre-profile and profile plates, unacceptable variations may still exist in the air temperature and velocity profiles.
In order to overcome these variations, the heated air is then forced to flow through a diffuser that consists of a perforated plate or woven wire having a large number of small holes therein. As the heated air passes through the small holes in the diffuser, it is subjected to a pressure drop and is intimately mixed so that the velocity and temperature profiles of the air flowing downstream of the diffuser is much more uniform.
Downstream of the diffuser, the air flows through a compression device which may be a conical or similarity shaped piece of ducting and which functions to increase the velocity of the air, typically by a factor of three, and to even out any remaining temperature and velocity irregularities that remain in the temperature and velocity profiles although its greatest effect is in smoothing out velocity differences. The heated air next flows through a lower air distributor which is a straight piece of ducting or tubing and which functions to straighten out any small variations in the air velocity profile that may still exist and to project and direct the air flow, at the desired velocity, into the drying chamber. To minimize turbulence caused by the nozzle atomizer of the atomizing system, a tapered aerodynamically designed piece is placed on and over the nozzle to streamline the shape of the nozzle and to allow air to flow past the nozzle with the least amount of turbulence.
In one embodiment of present invention described herein, the diffuser and the compression device are not separate components although the modified combined diffuser-compression device functions, for all practical matters, the same as the separate diffuser and compression device. In another embodiment, however, the compression device and its function are omitted while in still another embodiment, the lower air distributor, and a substantial portion of its function is accomplished by a modified compression device.
The air distribution system of our present invention also includes an internal air gap cooling system which is utilized to prevent the formation of a burned powder fringe at and around the interface between the drying chamber roof and the adjacent lower end of lower air distributor. This cooling system includes a ring that surrounds the interface and that has a relatively small annular gap to allow cooling air to flow into the drying chamber so as to cool the tip of the lower air distributor and the surrounding area. The employment of this cooling system allows the improved spray dryer of our present invention to be operated at relatively high air temperatures, without the formation of the objectionable fringe, and thus increases the efficiency and practical capacity of the spray dryer.
These and other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments of this invention, described in connection with the accompanying drawings.